Research of Materials Science June 2013, Volume 2, Issue 2, PP.28-33
The research and development of Fe doped TiO2 Mingqin Li Nv Zhen Road. SuiXi Economic Development Zone, Huaibei AnHui 235100, China Email: 563509617@qq.com
Abstract A comprehensive overview of research progress on the recent development of iron-doped TiO2 nanotube arrays modified method and application in photocatalytic degradation as well as dye-sensitized solar cells (DSSC). Meanwhile, the outlook of research and development on TiO2 nano-tube arrays were discussed. Keywords: Iron-doped; TiO2 Nanotube Arrays; Photocatalytic; DSSC
1 INTRODUCTION TiO2 is an important wide band gap semiconductor photoelectric conversion material. It has a unique optical, electrical and chemical properties, stable performance. What’s more, it can be able to resist electrochemical corrosion from medium, widely used in the semiconductor, sensor, dielectric materials, coatings, photocatalyst, cosmetics, as well as be used to make fire-resistant glass, glaze, enamel, clay, high temperature experimental containers. The TiO2 can disinfect and sterilize with UV. In a word, the research and development on TiO2 will become a new industry. Many applications of TiO2 thin film is at the basis of fundamental properties that relate to surface and interface features of the film [1]. Metal ion doping is to introduce metal ion into the inside of the TiO2 lattice structure by physical or chemical method, which result in introducing a new charge in the lattice, the formation of defects or a change in the lattice type[2]. Therefore, doping metal iron in TiO2 can change the transmission of photogenerated electrons and holes, and adjust their distribution state or the energy band structure. If an electron in the state of valence band gain the energy from light equal to or greater than the band gap of the semiconductor that it would be excited to the conduction band initiated. metal-ion dopants in the TiO2 nanocrystals work as recombination centers of the electrons and holes, which results in the decrease of activity of photogenerated electrons and holes to reduce the combination of electron-hole(fig..1). Since the first report about obtaining metal-ferromegnetic-semiconductor from Co-doped TiO2, there are many studies of the magnetic property of Fe-doped TiO2, because Fe and Co have the similar electronic configurations [3]. Fe-doped TiO2 has great potential application in spintronic and magneto-optic devices, many researchers has been focusing on systems formed by the presence of magnetic ions in non-magnetic matrices (diluted magnetic semiconductors (DMS) or diluted magnetic insulators (DMI) [4]. Noways the Fe-doped TiO2 film or power is mainly served as anode of Dye-sensitized solar cells (DSSC) or applied in photocatalytic degradation. The Fe-doped TiO2 appears apparent red-shift phenomenon and has higher photocatalytic activity in contrast with the original TiO2. In this essay, it reviews the Fe-doped TiO2 applied research achievements, analysis relational current problems, and put forward the outlook for future research.
- CB
CB I
hν +
VB
(a)
a
VB
+
(b)
FIG.1 DIAGRAM OF IRON-DOPED TIO2 ON PHOTOEXCITATION (a) ANDbRECOMBINATION (b) OF ELECTRON-HOLE; CB-CONDUCTION BAND, VB-VALENCE BAND, I-IRON - 28 http://www.ivypub.org/rms
2 PRINCIPLE OF DOPING MODIFICATION After understanding of the correlation between electronic structure and reactivity of a surface, one can change the surface by designing of novel catalysts for a broad range of reactions [5]. As to transition-metal surfaces, the d-band model provides a theoretical framework for understanding trends in reactivity from one surface to the next [6]. To grasp the theory of transition-metal reactivity, the electronic structure surface states (SSs) and the surface resonance states (SRs) of the compounds must be figured out [7]. The bulk and surface properties of TiO2 can be studied extensively using density functional theory (DFT) calculations [8,9]. It comes out that the oxygen interaction with all of the doped TiO2 surfaces is stronger than the one with undoped TiO2, and oxygen-surface bond gets weaker with increasing group number of the metal dopant [5]. The direct interfacial charge transfer (IFCT) from the valence band of TiO2 to Fe3+ would occur, which results in higher photocatalytic activity [10,11]. The TiO2 show a phase transition from anatase to rutile in the annealing process, and the phase change would be affect by surrounding environment [12].A system contains different phases TiO2 could separate electrons from holes to reduce electron−hole recombination [13].The phase transition behavior of TiO2 can be affect by many factors, such as initial grain size, temperature, chemical surroundings, and impurities [12]. Meanwhile, intragap can not only improve the surface activity of TiO2, but promote the nucleation of rutile crystallites forming between the adjacent anatase-rutile phase [12].
3 DOPING METHOD There are many ways to synthesis Fe-doped TiO2. However, most reports that study the Fe-doped TiO2 used the sol-gel method to introduce iron into TiO2, others are employed less, such as wet impregnation method[14], oxidative pyrolysis [15], plasma oxidative pyrolysis [16], and so on [2,17-24]. Followings will elaborate these methods in details.
3.1 Sol-gel The sol-gel method is the metal through the solution, sol, gel, and then solidify to obtain oxides or other compounds by heat treatment. According to sol-gel method, one can not only precisely control the chemical composition of the maerial, widen doping range, especially make lightly doped become easy, samples of good uniformity, high purity, but can be synthesized in high purity multi-component gel [25]. Bo Peng[26] et.al reported they used the general sol-gel method to get Fe-doped TiO2 with brilliant colors in the visible region and low reflectance in the ultraviolet range, which could have a bright future as pigments, ultraviolet prevention materials, and thermal conservation materials. While S.D. Delekar[27] put up that Fe-doped TiO2 synthesized using sol-gel method transform diamagnetic pure TiO2 into paramagnetic one. Meanwhile, they found that the higher iron content in host lattice, smaller optical band gap of titania nanoparticles decrease.
3.2 Sol-gel method combined with other methods According to using sol-gel methods with a combination of other methods, many dopant materials are prepared that can meet higher requirements and show better performance. Kais Elghniji[28] et.al. Synthesized Fe3+ doped TiO2 by the acid-catalyzed sol-gel method. The main merit of acidic catalysis is that it is possible to enhance simultaneously the crystallization of anatase phase and growth of brookite at low temperature [28]. The dopant synthesized by acid-catalyzed sol-gel method was characterized by Electron paramagnetic resonance (EPR) spectroscopy, the result show that Fe3+ ion in TiO2 plays a role as the intermediate for the efficient separation of photogenerated holeelectron pairs and increases the photocurrent response of the film under light irradiation(fig..2)[28] . Yean Ling Pang et.al[29] synthesized Fe-doped TiO2 nanotubes catalyst with large specific surface area and low band gap energy using sol-gel followed by hydrothermal method. The synthesized nanotubes had large specific surface area, high catalytic activity and sedimentation rate that resulted in high degradation efficiency under ultrasonic irradiation condition. Wenlan Wang et.al.[30] synthesized Er3+-Yb3+-Fe3+ co-doped TiO2 nanopowder, which applied to kill Aeromonas hydrophia under 980 nm laser irradiation(fig. 3). The co-doped TiO2 show strong penetrability of near-infrared, which may be used in photodynamic therapy (PDT) to kill bacteria or tumors directly, so as to be applied in medicine to contribute to human health - 29 http://www.ivypub.org/rms
3+
FIG.2 DIAGRAM ILLUSTRATING THE CHARGE TRANSFER FROM EXCITED TIO2 TO THE DIFFERENT STATES OF Fe
(a) undoped TiO2
ION
FIG. 3 TEM (b) Er3+-Yb3+-Fe3+ co-doped TiO2
3.3 Other methods X. H. Wang et.al got the dopant though oxidative pyrolysis of liquid-feed metallorganic precursors in a radiationfrequency (RF) thermal plasma. The RF thermal plasma could not only avoid postannealing that is essential in conventional method but well control the Fe/Ti molar ratios [31], because RF thermal plasma undergo high temperature and rapid heating and cooling rates , which make oxides nanopowders with good crystallinity possible to dope lightly and highly and dopant distributed in a very short time[15]. By doped in iron, the rutile TiO2 was strongly promoted, avoiding some defects introduced from the Fe3+ substituting for Ti4+ in TiO2. The oxygen vacancies reached a maximum at R(Fe/Ti)=2% above, the concentrated oxygen vacancies formed crystallographic shear (CS) structure as extended defect. What’s more, the dopant show manifest red-shift to extended the range of response to light. C.E Rodr´ıguez-Torres et.al used microemulsion method prepared Fe-doped TiO2, and characterized by XANES and EXAFS spectra at Fe K-edge (7112 eV) and Ti K-edge (4966 eV) at room temperature, the result show that iron K-edge near-edge and extended x-ray absorption fine structure confirm that Fe3+ replaces Ti4+ in the TiO2 anatase structure increasing the metal-anion bond length[18]. At low temperature, isolated paramagnetic Fe3+ ions and antiferromagnetically coupled Fe3+ ions can be distinguished. Doping causes an enlargement of cation–oxygen bond lengths around iron ions [18]. Jing Li et.al make the mixed aqueous solution of Fe(NO3)3-HF as electrolyte by means of anodic oxidation method synthesized highly ordered nanotubes[2](fig.4). Compared with nanopowder or nanoparticles, ordered nanotubes have larger surface area and absorb more dye as anode of DSSCs. ChinJungLin et.al synthesized mesoporous Fe-doped TiO2 sub-microspheres in a rapid and continuous aerosolassisted self-assembly (AASA) process with high surface area, accessibility, and crystallinity. In the process of doping, they successfully making the surfactant, titania, and the iron building clusters cooperatively assembled in - 30 http://www.ivypub.org/rms
one step [21].
FIG. 4 SEM (a) top view picture (b) cross-sectional view picture
FIG.5 SEM PICTURE (a)Low magnification SEM image of nanofibers before calcination; (b)High magnification SEM imagl; ofnanofibers after calcinadon at 500 ℃for 3 h
Yucheng Wu et.al Reported that University of Washington Li and XIA in 2003 synthesized TiO2 nanofibers using electrospinning method and the dopant with controllable diameter and porous structure of anatase phase (fig..5). In electrospinning method, one can adjust the proportion of titanium tetraisopropoxide salt and PVP, the concentration of ethanol solution, the auxiliary electric field strength, and the feed rate and other parameters to control diameter and porous structure of anatase TiO2 nanofiber [32]. Bin Lu Prepared Fe3+ doped TiO2 alcohol gel, combined with ambient drying, then obtained the Fe3+ doped TiO2 aerogels by atmospheric drying method [33]. He combined airgel high specific surface area and ion doping modification to greatly improve TiO2 photocatalytic properties, and use ambient drying instead of supercritical drying process, which lower preparation costs, shorten the preparation period. After characterized by XRD, BET, SEM, IR, the Fe3+doped TiO2 airgel with density of 0.24 g/cm3, the specific surface area of 529.17 m2/g, average pore size about 20.10 and its crystal was amorphous.
4 CONCLUSION AND OUTLOOK Compared with pure TiO2, the Fe-doped TiO2 has higher specific surface area, low combination of electrons and holes, and red-shift in light absorption[10,34-38]. Most dopants were papered by sol-gel method, although some of them combined with many other methods, morphology of sample is irregular. However, it is easy to doped all kinds of elements if needed, especially some lightly dope. what’s more, one can precisely control the chemical composition of the material by sol-gel method[25,39-40] . The dopants synthesized by anodic oxidation method can obtain highly ordered nanotubes, but the content of elements doped into TiO2 were very poor. Fe-doped TiO2 film or power is mainly applied in photocatalytic degradation or served as anode of Dye-sensitized solar cells (DSSC), or as pigments, and as disinfection and sterilization with UV. With the development of - 31 http://www.ivypub.org/rms
technology and increasing in demand for work or live, the TiO2 with special properties will play an important role in many fields. Someone has been trying applying TiO2 with special properties in (as) sensing material, light cracking water into hydrogen, lithium ion battery, Super-hydrophilic/superhydrophobic conversion film[41], Field emission device materials and so on [32]. The development of TiO2 would be a new industry, because it can be synthesized in many ways and appeared on many kinds of morphology, besides described in this essay, such as Sea urchin-like, dice-like, comb-like and so on.
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AUTHORS Mingqin Li (1986~), female, master, research fields: structure and properties of materials. Email: 563509617@qq.com
- 33 http://www.ivypub.org/rms